1. Field of the Invention
This invention relates to a variable-speed motor drive, and a method for operating a variable-speed motor drive, particularly to motor drives employing pulse-width modulation (PWM) driving methods, and especially to medium-voltage AC motor drives employing PWM driving methods.
2. Description of the Prior Art
In general, existing AC medium-voltage variable-speed drives for induction motors use a variation of current source topology, with a phase-controlled SCR input stage and a 6-pulse or 12-pulse output. This topology may sometimes have the drawbacks of harmonic line currents, a variable power factor, and motor torque pulsations. These traits are especially problematic at higher power levels typical for medium voltage motor drives. Because of these and other disadvantages of the current source topology, pulse width modulated (PWM) circuits are preferred to provide motor control. Pulse width modulation is a form of modulation in which the value of each instantaneous sample of the modulating wave is caused to modulate the duration of a pulse. In PWM, the modulating wave may vary the time of occurrence of the leading edge, the trailing edge, or both edges of the pulse. The modulating frequency may be fixed or variable.
In a PWM circuit, a reference signal may be used to generate a train of pulses, the width of each pulse being related to the instantaneous value of the reference signal. The pulses may be generated by using a comparator to compare the reference signal with a carrier signal, which may be a saw tooth or triangular wave. When the reference signal exceeds the carrier signal, the output of the comparator is high; at other times, the output of the comparator is low. The comparator output does provide a train of pulses representing the reference signal. The pulses are then used to drive an electronic switching device for intermittently applying a voltage across the load.
One problem associated with some PWM circuits is current ripple. When a voltage is suddenly applied across an inductive and resistive load, such as an electric motor, the current through the motor rises almost linearly with time. When the voltage is then turned off, the current through the motor does not immediately fall to zero, but decreases approximately linearly with time, as the inductor's magnetic field collapses. Thus, the input voltage pulses applied across the load result in a current which has a ripple. This ripple is inherent to all PWM amplifiers. The magnitude of the ripple is directly proportional to the supply voltage and inversely proportional to the switching frequency and the inductance of the motor. Current ripple is undesirable because it wastes energy in the motor and may cause torque pulsations in the motor. The ripple current waveform has both a wanted component and an unwanted component due to ripple. Only the wanted component does useful work in the motor. If the current has any ripple, the RMS value is larger than the wanted value. The difference between the RMS current and the wanted current contributes only to wasteful heating of, and torque pulsations in, the motor, thus reducing efficiency. In order to achieve maximum efficiency, the wanted and RMS currents generally should be equal. This conditions occurs when there is no ripple, i.e., when the output follows the reference exactly.
The electronic switches used in PWM circuitry can be arranged in a bridge circuit to connect the power source to the load. Typically, the bridges are connected such that, at certain moments, the respective contributions to the voltage applied to the load by the right and left bridge poles cancel each other, and, at other moments, the contributions have the same polarity. The net result is that the voltage applied across the load is zero during those moments when both bridge poles have the same polarity, and is equal to the supply voltage at all other times.